Atmospheric Pressure Ionization (“API”) ion sources are commonly used to ionize the liquid flow from high-performance liquid chromatography (“HPLC”) and higher pressure chromatography devices prior to analyzing the resulting gas phase ions via a mass spectrometer. Two techniques which are most commonly used comprise Electrospray Ionization (“ESI”) and Atmospheric Pressure Chemical Ionization (“APCI”). ESI is optimal for moderate to high polarity analytes and APCI is optimal for non-polar analytes. API ion sources that combine both of these techniques have been proposed and realized in designs that simultaneously combine ESI and APCI ionization. Such “multimode” ion sources have the advantage of being able to ionize analyte mixtures containing a wide range of polarities in a single chromatographic run without the need to switch between different ionization techniques. Surface Activated Chemical Ionization (“SACI”) is another type of ion source which directs a vapor stream from a heated nebulizer probe towards a broad area charged target plate which is situated close to the ion inlet aperture of the mass spectrometer. The spray point of the SACI ion source is within the heated nebulizer probe and is usually situated so that a relatively large distance exists between the sprayer and the target. Such distance produces a divergent spray with a dispersed reflected flow at the target, which generally results in lower sensitivities when compared to optimized ESI and APCI sources.
As described above, a SACI ion source converts a liquid stream into a vapor stream that then impinges on a broad area target. U.S. Pat. No. 7,368,728 discloses a known SACI ion source and is incorporated herein by reference in its entirety. Experiments on SACI (Cristoni et al., J. Mass Spectrom., 2005, 40, 1550) have shown that ionisation occurs as a result of the interaction of neutral analyte molecules in the gas phase with the proton rich surface of the broad area target. In contrast to SACI, a pneumatic nebulizer used for impact spray ionization utilizes a smaller target and emits a high density droplet column. Experiments involving pneumatic nebulizer ion sources (Bajic, WO/2012143737 published Oct. 26, 2012, incorporated herein by reference in its entirety) that utilize a streamlined target to intercept a high velocity stream of liquid droplets, which results in a secondary stream of secondary droplets, gas phase neutrals and ions, have demonstrated that such a technique can result in spray that is highly collimated with greater than two thirds of the total droplet mass of the spray being confined to a radius of 1 mm from the nebulizer or sprayer. However, an observed loss of sensitivity at lower flow rates makes these techniques undesirable for many applications. Use of pneumatically assisted nebulizers for producing an impacting spray is also well known in the art. This class of nebulizers is known to have the undesirable property of producing variably-sized droplets as the flow rate of the liquid stream to be nebulized decreases or drops. Therefore, there is a need in the art for an ion source for a mass spectrometer that improves sensitivity.